package offer.diff;

/**
 * 数字以 0123456789101112131415... 的格式序列化到一个字符串中，求这个字符串的第 index 位
 * @author DengYuan2
 * @create 2021-04-20 8:53
 */
public class n_44 {
    public static void main(String[] args) {
        n_44 n = new n_44();
        int res = n.getDigitAtIndex(1245);
        System.out.println(res);
    }

    public int getDigitAtIndex(int index) {
        if (index < 0)
            return -1;
        // 1 表示个位，2 表示 十位...
        int place = 1;
        while (true) {
            int amount = getAmountOfPlace(place);
            int totalAmount = amount * place;
            if (index < totalAmount)
                return getDigitAtIndex(index, place);
            index -= totalAmount;
            place++;
        }
    }

    /**
     * place 位数的数字组成的字符串长度
     * 10, 90, 900, ...
     */
    private int getAmountOfPlace(int place) {
        if (place == 1)
            return 10;
        return (int) Math.pow(10, place - 1) * 9;
    }

    /**
     * place 位数的起始数字
     * 0, 10, 100, ...
     */
    private int getBeginNumberOfPlace(int place) {
        if (place == 1)
            return 0;
        return (int) Math.pow(10, place - 1);
    }

    /**
     * 在 place 位数组成的字符串中，第 index 个数
     */
    private int getDigitAtIndex(int index, int place) {
        int beginNumber = getBeginNumberOfPlace(place);
        int shiftNumber = index / place;
        String number = (beginNumber + shiftNumber) + "";
        int count = index % place;
        return number.charAt(count) - '0';
    }
}
